1. Field of the Invention
This invention relates to a gas mixture for use in formation of ball bonds to semiconductor devices, particularly in conjunction with copper, aluminum and gold wire and, more particularly, to the atmosphere utilized during formation of such bonds.
2. Brief Description of the Prior Art
In the formation of wire bonds to the bond pads of semiconductor device, it is standard procedure to provide a capillary tube through which gold wire to be bonded to the bond pad on the semiconductor device is threaded. The wire is generally connected to a positive or negative reference voltage source. An EFO (electronic flame off) or torch, which is coupled to a source of voltage of opposite polarity to that of the wire, and which is generally formed of tungsten, is positioned closely adjacent the lower or exit tip of the capillary tube. To provide the bond, an arc is struck between the EFO and the portion of the wire extending out of the capillary tube while a predetermined atmosphere formed around the exposed wire is controlled by blowing said atmosphere around the exposed wire to first form a ball due to the surface tension and then provide the bonding step with the ball. The prior art atmosphere used was generally a reducing gas, preferably hydrogen, and an inert gas, preferably argon and was directed from a tube within a few mils of the electrode gap and around the exposed wire.
For several reasons, primarily that of cost, it has been a desire in the art to replace the gold bonding wire with a less expensive substitute, copper and aluminum being the materials of primary interest due to their good electrical conductivity and low cost. However, the basic nature of gold and its ease of working and adaptability to the semiconductor bonding process have made it difficult to replace.
In general, the major problems encountered in morphological control, primarily of aluminum wire and also of copper wire are skew or centering of the wire on top of the ball, optimum size control for compatibility with pad size and capillary and necking down of the wire above the ball.
Gases of various combinations, but mainly of a reducing or inert nature have been used most successfully. The overall problem of skew or lack of symmetry between the aluminum wire and ball center is due mainly to the surface tension resulting from the aluminum oxide formation after melting when the wire tip forms a ball. In addition, all other factors being fixed, the breakdown voltage, V.sub.b, is directly dependent upon the gas atmosphere of the arc and the electrode spacing. This is essentially Paschen's Law, where V.sub.b =nd, and n=gas density (or a function of pressure from ideal gas law assumption) and d=electrode spacing. However, the law will show deviations for impurities, gas mixtures and electrode material variations. In particular, where inert gases are involved with metastable states, very low V.sub.b 's are possible. This is a consequence of the Penning effect and their low electric strengths result from the absence of low energy excited states.
Various gases and combinations of gases used in aluminum and copper ball formation in the prior art were nitrogen, argon and hydrogen, neon and argon, argon and water vapor and others. Argon appears to be the most useful and practical and, when combined with hydrogen in small amount (about 1% hydrogen), provides the best ball formation. The advantages of using such a mixture over argon alone are several and highlights of the mechanisms are described hereinbelow.
The dissociation of molecules is a predominant mechanism in heat transfer in the arc. Energy absorbed from the arc in dissociation is released when they are reformed (such as N.sub.2 =2N). The temperature (energy) available for melting at a surface is, therefore, increased by the heat of dissociation. Improved heat transfer by atomic species is advantageous in the melting processes resulting from arc generation. Argon, for example, has no dissociation processes and results in relatively poor heat transfer or only those of conduction and radiation. Heat transfer by diffusion of dissociated atoms accounts for a large part of the arc's useful temperature and would seem, at first, to be the more useful process. Some compensation can be obtained, however, while still using argon in combination with other gases. For example, it is also observed in the EFO arc that argon alone does not provide the best morphology observed in aluminum ball formation. This is partially due to aluminum oxide formation from trace oxygen from the air which is probably always present around the electrodes. When some hydrogen is present in the argon, the power required to create the arc and form the ball is considerably less (about 15 to 20%). The hydrogen also reduces the oxide formation on the aluminum or is oxidized itself, thereby eliminating some available oxygen. In any event, the hydrogen minimizes the surface tension of the aluminum ball exterior and aids in the formation of smooth, symmetric balls. Additionally, the argon-hydrogen mixture results in lowering of the breakdown voltage and improves gaseous conductivity, presumably via the Penning effect, which causes some ionization of the metastable argon. This can be very beneficial in the EFO process.
Inert gases have high ionization potentials or require relatively high energies to remove their outer electrons. However argon, for example, can be relatively easily promoted to a metastable excited state, excitation being removal of the outermost electrons at a distance but not beyond nuclear influence. The collision reaction involving the metastable, excited species, results in easier ionization than for normally excited atoms. By strict definition, "metastable" refers to energy levels from which the excited electrons cannot return to the ground state directly by emission of radiation. Lifetimes of these states are long compared to other types of excited states. The longer lifetime of an atom in a metastable state results in a pickup of sufficient additional energy when colliding with an electron to cause ionization, even though the electron does not have enough energy to ionize an atom in its ground state. The result is that ionization can occur in argon, even though the discharge potential may be lower than the ionization potential.
The presence of metastable atoms has a considerable effect on gas conductivity if only a small amount of atoms of a different element having a lower ionization potential than the metastable are present. In the case of argon and hydrogen, where argon is only slightly higher in ionization potential, the argon should produce ionization of the hydrogen particle. Because of this Penning effect (formerly called "collisions of the second kind") and the relative energy relationships in operation here, increased and sustained conductivity can be more easily maintained than by use of a monoatomic gas alone. It appears, therefore, that the presence of hydrogen is advantageous and practical in the EFO arc for several reasons.
It is also well observed that too much hydrogen causes high porosity as with water in argon. It has been long considered that molten aluminum absorbs hydrogen preferentially and that this is due to the reduction of water by the aluminum and freeing of hydrogen which goes into the melt and forms voids as its solubility lessens upon solidification of the melt. It is probable that the better skew behavior, shape factors and even more controlled ball formation, aside from the void problem, are due to a slower cooling resulting from the formation of boehmite (AlOOH) and gibbsite (Al(OH).sub.3) rather than Al.sub.2 O.sub.3 only from a dry oxidation. These two hydrated oxides have higher thermal expansion and less accommodating structures to fit the aluminum surface as it solidifies. Work done with argon having from 4% hydrogen to up to 10% hydrogen has been found to produce high porosity and variable ball size. It is therefore apparent that these prior art gas mixtures have demonstrated problems, especially in conjunction with bonding of aluminum wire to semiconductor device pads.